The invention relates to an inverter circuit in which the energy is largely recovered in the electrical oscillations produced during switching operations, and also comprises a method for regeneratively damping electrical oscillations which is implemented with this inverter circuit.
Inverters convert a DC voltage provided by a source into an AC voltage of square-wave pulses of variable frequency which is then made available to a load, for example for operating an electrical machine, for instance for driving a motor for an electric vehicle.
Inverters are used in a single-phase and multi-phase form. In the case of a single-phase inverter, the outputs are alternately connected to the positive and negative pole of the source via switches. In the case of a multi-phase inverter, the polarity of the individual terminals of the load (phases) is reversed in a sequence which depends on the application. For example, a three-phase inverter can be used to cyclically reverse the polarity of three windings of a motor which are interconnected in star. In another example, a three-phase inverter can be used to cyclically reverse the polarity of three windings of a motor which are interconnected in delta.
FIG. 6 shows the basic circuit of a three-phase inverter according to the prior art. Here, the DC voltage is provided by a rechargeable battery and is made available at an intermediate circuit capacitor. Each of the three phases can be separately connected to the positive pole or the negative pole of the DC voltage via a respective semiconductor switch, MOSFETs or IGBTs each with diodes (freewheeling diodes) connected in parallel preferably being used as semiconductor switches. In this case, the two switches of each phase are operated in the push-pull mode, but with a certain dead time during switching in order to reliably avoid overlaps which would certainly signify a short circuit of the source.
At the moment of opening the semiconductor switches, the parasitic inductances of the circuit and here, in particular, the inductances of the connecting lines between the source with the intermediate circuit capacitor and the power module with the semiconductor switches become disruptively noticeable. This is because the energy stored in these parasitic inductances causes an induction voltage surge which results in overvoltages at the semiconductor switches and thus jeopardizes the power module and also results in electrical oscillations. In order to counter this, the prior art provides two different procedures which can also be combined: on the one hand, the electronic switches are designed from the outset for the expected overvoltages by virtue of their breakdown voltage being considerably above the intermediate circuit voltage. On the other hand, an additional capacitor can be connected to the semiconductor switches in a low-inductance manner (“capacitive snubber”), in which case “low-inductance” means that it should be integrated in the power module while minimizing the supply line lengths. In this case, an electrical oscillation is produced between this capacitor and the parasitic inductances, which oscillation is damped by the non-reactive resistors of the circuit (not illustrated in the figure) and decays. In both cases, the magnetic energy stored in the parasitic inductances is converted into heat during each switching operation and results in losses which increase in a manner proportional to the switching frequency.